There are various types of wind turbines on the market. Of these, the horizontal axis wind turbine, or ‘HAWT’, is probably the most common and comprises a nacelle on top of a vertical tower. The nacelle supports a rotor having a set of blades which rotate about a horizontal axis.
For performance reasons it is important that the rotor is in alignment with the wind direction. In some HAWT designs the rotor is designed to be operated facing the wind, and in others the rotor is designed to operate facing in a ‘downwind’ direction.
In both of these configurations, the wind turbine includes a nacelle yaw control system which is operable to yaw the nacelle about the tower axis. The yaw control system uses wind direction information, for example from a nacelle-mounted anemometer and direction sensor to determine the wind strength and its direction relative to the yaw position of the nacelle. The yaw control system then is able to yaw the nacelle so that it faces into the wind, thereby maximising the power that the wind turbine is able to extract from the wind.
In performing the yaw control of the nacelle, the yaw control system must be provided with accurate position information about the nacelle. Without accurate information, there is a risk that yaw control system is unable to align the nacelle with the wind direction which may lead to a reduction in power generation efficiency. Also, it is important nowadays for a wind turbine to be able to determine the absolute wind direction, as opposed to the direction relative to the nacelle, since this parameter may be used for certain control strategies, such as wake reduction techniques for example. Absolute wind direction is usually determined by adding relative wind direction to the nacelle yaw position.
In a typical yaw control system, the nacelle position is derived from a sensor that monitors the movement of a tower-mounted yaw ring gear that is rotated by one or more motor-driven yaw pinions. Usually an optical or magnetic encoder sensor is used for this purpose. Rotational movement of one of the yaw drive pinions may also be measured.
Errors in the yaw measurement system can in certain circumstances accumulate into a significant yaw error which has a knock on affect particularly in the determination of a value of the absolute wind direction for the wind turbine. It will be appreciated from the above discussion that it would be desirable at least to detect when a yaw sensor is providing inaccurate data so that appropriate action can be taken. It would also be desirable to mitigate for the presence of the inaccurate data so that a given wind turbine in a wind farm always has an acceptably accurate measure of absolute wind direction available to it so that control objectives can be achieved.
It is against this background that the invention has been devised.